1. Field of the Invention
The present invention relates to an electromagnetically actuated pressure-regulating valve.
2. Description of Related Art
In modern passenger-car automatic transmissions, hydraulically actuated clutches are used for changing gears. In order for the shifting operations in the automatic transmissions to proceed smoothly and imperceptibly for the driver, it is necessary to adjust the hydraulic pressure at the clutches with the highest pressure precision, in accordance with predefined pressure ramps. Electromagnetically actuated pressure-regulating valves are used for adjusting the pressure ramps mentioned. The pressure-regulating valves are generally in a seat type of construction or valve-piston type of construction. The pressure level required is achieved via a pressure balance integrated in the valve, the force of the electromagnet, changeable as a function of current, being brought into equilibrium with the hydraulic force on the valve seat.
In order to achieve the requisite pressure precision, it is necessary that the magnetic force, changeable by the coil current, take a course corresponding to an exact characteristic curve. Mechanical friction within the electromagnet—particularly in the case of the armature bearing—leads to hysteresis in the magnetic force and therefore to inexactness in the regulating pressure.
For cost reasons, electromagnets are used today for the applications with respect to passenger-car automatic transmissions indicated above, in which the component for the radial infeed of the magnetic flux into the armature (magnet core) and the component representing the complementary magnetic pole for the magnet armature (pole body) are combined to form one component known as a pole tube. To avoid a magnetic short-circuit in a pole tube, for instance, a V-shaped groove is introduced into this component. The magnetic iron cross-section reduced in this manner in the groove already enters a state of saturation in response to low coil currents, and thereby acts like a separating air gap. Ideally, in these pole-tube designs, the armature is supported directly in the pole tube, so that little expenditure is necessary for additional bearing components. Moreover, because of the small air gaps, a high magnetic efficiency and consequently a high magnetic force may therefore be attained.
However, the disadvantage in this embodiment variant of the bearing assembly is the circumstance that relatively high magnetic transverse forces develop, which lead to increased friction and therefore hysteresis and inexactness of pressure. In order to offset this disadvantage, in the known examples, coatings are used which act to reduce friction and which provide for a magnetic separation in the secondary gap between armature and pole tube. As a rule, however, these coatings are costly to produce because they require the handling of individual parts during the coating process as well as, in some instances, a mechanical postprocessing of the coating to attain the necessary geometrical accuracy. In addition, the coatings used under the state of the art do not achieve the optimal coefficient of friction which, for example, could be achieved by the use of a Teflon bearing.
Published U.S. patent application 2004/0085169 A1 discloses an electromagnetic actuator including a sliding section, i.e., an armature and a stator. The stator together with the sliding section, the armature, form a magnetic circuit. To accommodate and support the armature, the stator includes an accommodation section within which the armature is able to move back and forth. A coil generates a magnetic force for displacing the armature in one of the two directions. To reduce the friction between the stator and the sliding section, the location hole for accommodating the armature is coated with nickel phosphate, the phosphorus content varying in a range between 5 and 15% by weight.